Polyethylene terephthalate is prepared on an industrial scale by one of two processes. The first is the terephthalic acid or TA-process. In this process, terephthalic acid is reacted with ethylene glycol in a direct esterification reaction, producing bis-(2-hydroxyethyl) terephthalate (commonly designated as monomer) and water. The reaction is reversible, hence the water formed during the reaction has to be removed in order to convert the starting materials completely into the monomer. A catalyst is not required in this reaction and conventionally no catalyst is employed. The monomer then undergoes polycondensation to form the PET. In the polycondensation reaction typically antimony catalysts are employed (see e.g. U.S. Pat. No. 5,153,164).
In the dimethyl terephthalate or DMT process, the dimethyl terephthalate is reacted with ethylene glycol at a temperature of between 150 and 250.degree. C. at approximately atmospheric pressure in an ester interchange reaction (transesterification) to yield bis-(2-hydroxyethyl) terephthalate and methanol. The ratio of ethylene glycol to dimethyl terephthalate is in general between 1.4:1 and 2.5:1, preferably between 2.0:1 and 2.5:1. The most typical catalyst employed in the transesterification reaction is a manganese catalyst, specifically manganese acetate. In addition, catalysts based on other metals, such as magnesium, lithium, calcium, cobalt or zinc, may also be used alone or in combination. The transesterification reaction is reversible and the reaction equilibrium can be forced towards the bis-(2-hydroxyethyl) terephthalate only by removing the methanol from the reaction mixture. At the end of the transesterification reaction the activity of the catalyst is sequestered and inactivated by introducing phosphorus. If the catalysts employed in the transesterification reaction are not arrested with phosphorus, the resultant polymer easily degrades upon heating and shows an unacceptable yellow color. For example, this is set forth in the article of S. G. Hovencamp, "Kinetic Aspects of Catalyzed Reactions in the Formation of Poly(ethylene terephthalate)", published in J. Polymer Sci., Part A-1, vol. 9 1971!, pages 3617-3625.
In the second stage of the conventional batch process, the bis-(2-hydroxyethyl) terephthalate is polycondensed and thus converted into PET. Polycondensation is carried out generally at a pressure in the range of from about 0.01 to 40 mm Hg (about 0.01 to 53 mbar) at a temperature in the range of from about 205 to 305.degree. C. The reaction time is about 1 to 4 hours. The catalyst most frequently employed in the polycondensation reaction is a catalyst based on antimony, such as antimony acetate or antimony trioxide. Additives may be added during the process, for example, antiblocking agents, brighteners, bluing agents, color pigments and opaquing agents.
The conventional continuous process is basically an extension of the batch process in which a series of two or more sequentially connected vessels are used to perform the stages of transesterification and polycondensation. In general, a continuous feed of starting materials is used and the methanol formed is removed as the reaction proceeds, thus enabling the reaction to go to completion. The first vessel in the process is generally at atmospheric pressure and the pressure in each successive vessel is decreasing until in the final polycondensation vessel the pressure is generally in the range of from 0.01 to 40 mm Hg (=0.01 to 53 mbar). The temperature through the series of vessels generally increases from about 150 to 290.degree. C. in the first vessel to 200 to 305.degree. C. in the last vessel. The increase in temperature and the decrease in pressure aid in the removal of excess ethylene glycol. The flow of the reactant stream is continuous through each vessel and from vessel to vessel. In addition, the output of PET is continuous. In the continuous process, the same catalysts and sequestering agents can be used as in the batch process.
EP-A 699 700 discloses a process for the production of polyesters from dicarboxylic acid dialkyl esters by (i) transesterification with an aliphatic diol in the presence of a metal catalyst, in particular a manganese catalyst, (ii) adding an agent based on phosphorus which complexes and inactivates the transesterification catalyst, (iii) adding a cobalt compound and (iv) polycondensing the intermediate product in the presence of 1 to 10 ppm of titanium and in the absence of antimony.
U.S. Pat. No. 3,907,754 teaches a process for the production of PET in which the transesterification reaction between dimethyl terephthalate and ethylene glycol is conducted in the presence of a catalytic amount of organic or inorganic salts of manganese and cobalt with acetyl triisopropyl titanate, and subsequently a phosphate ester is added before polycondensation. According to U.S. Pat. No. 4,010,145, an antimony compound is additionally present during the transesterification reaction.
U.S. Pat. No. 4,122,107 discloses a catalyst which is useful both in the transesterification and in the polycondensation step. The catalyst is a bimetallic compound obtained by reacting (1) a first compound selected from the group consisting of antimony(III) salts of monocarboxylic and dicarboxylic acids, antimony(III) and zirconium(IV) alkoxides and antimony trioxide, (2) a second compound selected from the group consisting of zinc, calcium and manganese salts of mono- and/or dicarboxylic acids, and (3) at least a stoichiometric amount of a third compound selected from the group consisting of anhydrides of mono- and dicarboxylic acids, alcohols containing 1 to 20 carbon atoms and glycols containing 2 to 20 carbon atoms. The molar ratio of the trivalent antimony or tetravalent zirconium compound to the manganese, zinc or calcium salt is from 1:1 to 1:6. The catalyst is employed in an amount of 0.01 to 0.3% by weight, based on the weight of the DMT. The PET produced by this process shows a relatively high number of particles or residue produced from the catalyst system. This is a severe disadvantage in many applications.
The problem to be solved is hence to improve the DMT process in order to decrease the number of particles in the PET, and similar polymers, which are produced from the catalyst system. An article produced from the PET accordingly should have a very smooth surface which shows only a minimum number of protrusions.
It is an object of the present invention to provide a process for the production of PET, and similar polymers, starting from di(C.sub.1 -C.sub.4)alkyl terephthalates, or similar compounds. These terephthalates, or similar compounds, are reacted with ethylene glycol (EG), or similar glycols, yielding bis-(2-hydroxyethyl) terephthalate, or similar compounds, which is then polycondensed to form PET, or similar polymers.
It is another object of the present invention to provide one or a plurality of shaped articles made of PET, or similar polymers, and in particular to provide a biaxially oriented PET film, or similar polymer film, which shows a significantly reduced number of particles in the polymer matrix and, accordingly, also has a very smooth surface with very few protrusions or other surface imperfections.